The present invention is in the field of screen printing and curing. The present invention is more particularly in the field of safety devices designed for used in the curing process during screen printing.
In the field of screen printing (previously referred to as silkscreen printing), machines known as flash cure units are used to quickly cure ink deposited on a substrate during screen printing. The cure units provide curing radiation via a radiation source such as ultraviolet or infrared lamps. In practice, the substrate being screen printed is typically positioned on a platen of a screen printing machine for proper application of ink. Then the screen printing machine is repositioned so as to place the platen holding the printed substrate under the cure unit to quickly cure the ink. Speedy ink curing is especially important in multi-color printing since a first colored ink must be at least somewhat cured prior to applying another colored ink, in order to avoid mixing and smearing of the various colored inks.
Screen printing cure units are categorized as automatic systems and manual units. As used herein the term xe2x80x9cautomaticxe2x80x9d refers exclusively to the fact that the cure unit is controlled by the screen printing apparatus. For example, some automatic systems are designed so that voltage is increased to the cure unit upon a particular indexing positionment of the printing apparatus and then the voltage supplied to the cure unit is automatically reduced upon an alternative indexing positionment of the printing apparatus. In contrast, a xe2x80x9cmanualxe2x80x9d cure unit is not controllably connected to the screen printing apparatus. A manual cure unit is supplied with a continuous single-voltage power supply and emits a constant flow of curing radiation. For clarity, the categorization of a cure unit as being automatic or manual is independent of whether the screen printing apparatus is a type which is automatically indexing or repositioning or of a type which must be manually repositioned by an operator.
Operators of screen printing equipment have encountered the problem of the cure unit applying heat to a printed substrate for too long of a period. The application of too much heat firstly results in scorching of the substrate. But the most dangerous risk posed is the pervasive risk of ignition of the substrate, thus posing a real danger to the entire work facility.
While safety devices have been incorporated into automatic screening/curing systems, no safety device has heretofore been offered for use with a traditional manual cure unit. Manual ink curing inherently poses a higher risk of ignition than does automatic curing. A screen printing operation using manual curing involves intermittent periods where the cure unit is not being used to cure deposited ink, yet it is emitting a full stream of curing radiation. Additionally, manual curing is often used in retail stores or small home-based shops where articles such as T-shirts are custom printed. In practice, the workspace devoted to the printing operation is often small and cluttered. It has been found that foreign objects are more likely to inadvertently enter into the dangerous scorch zone of the stream of radiation emitted from a manual cure unit than with an automatic cure unit. Accordingly, there is a need in the art for a safety device to prevent scorching and ignition of printed substrates and foreign objects from manual cure units.
Automatic screen printing systems incorporating automatic cure units have been designed to incorporate safety means for preventing excessive application of heat to the printed substrate during curing. U.S. Pat. No. 5,249,255 (Fuqua) discloses a similar printing system equipped with a safety time limit switch that causes the high voltage supplied to the cure unit to be reduced to a much lower voltage after the cure unit has been operated at its high voltage for a predetermined maximum amount of time. Such safety means regulated by the length of time that the cure unit has been operated between cure cycles would not be suitable for a manual cure unit since a manual cure unit is powered by a constant flow of electrical power and has a constant output of radiation.
U.S. Pat. No. 4,517,893 (Wile) discloses a printing system with an automatic cure unit designed so that the heating component of the cure unit is automatically retracted away from the printed substrate if the operator fails to index the screen printing apparatus within a certain period of time from the previous indexing of the screen printing apparatus. This requires a timer that is actuated and reset upon each indexing of the printing machine. Such safety means would not be suitable for a manual cure unit since a manual cure unit is not controllable by the movement of the screen printing apparatus.
U.S. Pat. No. 5,218,908 (Whitfield) discloses a manual cure unit for curing a printed ball cap. This particular manual cure unit is equipped with a slideable tray upon which the ball cap is positioned during curing. The manual cure unit is adapted so that the tray engages to actuate a timer when the operator slides the tray and cap inwardly toward the lamp. Upon reaching the maximum pre-selected time period, the timer actuates a lever to push the tray away from the heat lamps. A manual cure unit having such engaging and actuating parts would be expensive and would deleteriously limit the possible uses of manual cure units. The requirement of contacting and positioning the workpiece on a component of the cure unit is not suitable for high-volume screen printing and curing systems where the manual cure unit is used as an integral part of a multi-color printing process.
In light of the above, it would be desirable to provide a new safety device for ensuring that a manual cure unit does not emit an excessive amount of radiation onto a substrate during curing. Such safety device should be suitable for use with a manual cure unit during high volume multi-color screen printing processes. It would be further desirable for such safety device to be operative without requiring any operator action between curing each item. It would be even further desirable for such safety device to be operative without any contact between the printing machine and the cure unit or between the printed substrate and the cure unit. Further still, it would be desirable for such safety device to operate while the cure unit remains in a static position, without any pivoting or retracting motion being required. Such a safety device would be suitable for use with the traditional free-standing, rigid manual cure units widely used in the screen printing industry. Each of the objects of the present invention is to provide a safety device having these desirable features and benefits.
The present invention comprises a safety device for use with a manual cure unit during a screen printing process. The safety device is useful in conjunction with a cure unit that has a radiation source that emits a constant stream of radiation defining a scorch zone. The radiation source is powered by a continuous supply of electric current having a voltage. The present safety device comprises the following components:
a) a proximity sensor positionable at a location proximate the scorch zone, said proximity sensor having a capability to detect the presence of an object disposed in the scorch zone when positioned at said location, and wherein said proximity sensor is adapted to provide a continuous current through an output line while detecting an object in the scorch zone;
b) a voltage reduction mechanism interruptably connectable to the continuous supply of electric current of the cure unit and adapted to reduce said voltage upon actuation, wherein said mechanism actuates upon receiving current through an input line to said mechanism; and
c) a timer receivably connected to said output line of said proximity sensor and providingly connected to said input line of said voltage reduction mechanism, wherein said timer is programmable to measure the duration of time that current is continuously received from said output line of said proximity detector and, upon measuring a pre-selected maximum safe cure time, to provide a current to said input line of said voltage reduction means, thereby causing said voltage to be reduced.
In an alternative embodiment of the present safety device, the voltage reduction mechanism is substituted with an alarm so that the safety device is useful to alert the print operator to manually remove the object from the scorch zone. The most preferable embodiment of the present safety device comprises both a voltage reduction mechanism and an alarm for optimum benefits.